KerrBH spin parameter has wrong restrictions

[mirenradia]

Thanks to @TaigoFr for spotting this.

Currently the KerrBH initial conditions compute class only allows spins between 0 and 1 (see here). This is correct if the mass, M = 1 but since the spin provided is the dimensional spin, commonly denoted a, (with units of mass in geometric units) which has an upper bound of M, this is of course wrong if M ≠ 1.

We could change to using the dimensionless spin (i.e. a/M) or we could just change the check above to allow spins in the range [0,M].